The invention relates to linear analog to digital converters and further to the use of such a converter in an apparatus and method for measuring repeatedly the absorption of electromagnetic radiation by a plurality of specimens over a period of time. More particularly this invention concerns an apparatus and method in which each of a plurality of fluid samples or aliquots (portions of the samples) in reaction vessels or cuvettes is subjected to chemical reaction with different reagents. The electromagnetic transmittance of each aliquot repeatedly is determined during the reaction period. The measurement herein involves accurately ascertaining the value of electromagnetic radiation transmittance at a particular wavelength by the fluids in the cuvettes and converting the signals from an analog to a digital form so that digital transmission, conversion to absorbance data, storage and processing may occur.
It is desirable to make such an analysis on a continuous process in which the apparatus continues to operate as long as there are samples to be tested, the old samples and their tested aliquots being replaced by new samples and their aliquots without interruption of the operation of the testing apparatus. Such continuous operation includes one or more photometric measurements on a given aliquot by one or more photometers. It is preferably that the analog transmittance signals received each time a reaction vessel passes through a light beam be converted into digital transmittance signals that thereafter are transmitted to a central processor where they are converted into digital absorbance signals.
The continuous analyzers of interest typically supply sample portions to reaction vessels which are monitored by measuring the transmission of light by the fluids in the cuvettes at a particular wavelength or wavelengths. Sample fluids placed in cuvettes typically are body fluids of a specific patient with one or more tests related to the patient's condition of health being conducted. It is therefore critical that the signals obtained from the fluids in the cuvettes be both accurate and repeatable. The sampling of the transmittance signals and therefrom the production of the absorbance data of each aliquot should be precisely repeatable for each cuvette and each light beam passing through the cuvette.
Analog to digital (A/D) conversion of electrical signals such as the transmittance signals herein is well known. In a simple single-ramp A/D converter the unknown voltage level signal, the level of which is to be digitized, is applied to one of a pair of inputs of a comparator. A fixed or constant slope ramp signal from an integrator is applied to the other comparator input. A constant frequency clock signal is gated to a counter at the beginning of the ramp signal slope and is stopped when the ramp signal equals the voltage level of the unknown voltage level signal, as determined by the comparator. The count in the counter then represents the digital conversion of the unknown voltage level.
Such a single-ramp type of converter is one of the simplest circuits available for performing the A/D conversion. It suffers, however, from several errors that affect its accuracy: the slope of the ramp signal may vary over each cycle; the frequency of the clock signal may vary; and the comparator may introduce propagation delay intervals and offset voltages.
Thus, in applications involving A/D converters, the two words "simple" and "accurate" usually are contradictory. The simpler the circuit the less accurate, and the more accurate the circuit the less simple. The word "accurate" generally refers to longer data words representing the desired information; thus a 16 bit data word is more accurate in representing voltage values of a range of say 0-10 volts than a 4 or 10 bit data word.
Motorola Application Note AN-559, "A SINGLE RAMP ANALOG TO DIGITAL CONVERTER", Jim Barnes, Applications Engineer describes modifications to overcome some of the inaccuracies introduced by a single-ramp A/D converter. One described circuit provides twin comparators. One senses the ramp signal slope passing through a start level at which time the clock frequency is gated to the counter. The other senses the ramp signal equaling the unknown voltage level signal to stop the counter. This reduces ramp start and stop time variations caused by comparator offset voltage and propagation delay errors.
Another described circuit performs a comparison to a known standard for each measurement cycle. A known voltage level reference signal is applied to the A/D converter and a digital result is obtained. The digital result is compared to the known correct result for the reference signal. The difference between the obtained and known result is used to adjust the frequency of the clock signal that is gated to the counter. After adjustment, the reference signal is switched from the converter and the unknown voltage level signal is applied thereto and digitized.
In effect, this Application Note indicates the need to provide additional circuitry to overcome the inaccuracies introduced by the simple single-ramp A/D converter.
Multiple ramp A/D converters also are available to provide increased accuracy.
Dual-ramp A/D converters are especially suitable in digital voltmeters and those applications in which a relatively lengthy time may be taken to obtain the benefits of noise reduction through signal averaging.
Such a converter operates by applying the unknown signal to an integrator and starting a counter counting clock pulses. After a fixed interval of time, a reference voltage having opposite polarity is applied to the integrator and the counter again begins counting from zero. When the integrator output reaches zero the counter is stopped and its output is the binary representation of the input voltage. A circuit diagram of such a dual ramp circuit is located in "Analog-Digital Converter Notes", Analog Devices, Inc., 1977, Chapter 11-1, page 123.
A shortcoming of conventional dual-ramp converters is that errors introduced at the input to the integrator or comparator become errors in the digital count. Such errors may be reduced by charging a capacitor with such zero-drift errors during a third cycle of the measurement and introducing the stored charge in the opposite sense during the integration interval to nullify the errors.
U.S. Pat. No. 3,872,466 discloses a quad-ramp circuit for reducing errors due to offset voltages in A/D converters. An integrator is ramped up to and down from a reference level during a calibrate operation by sequential application of opposite polarity reference signals. A digital determination of net offset error is made by comparing the total time of ramp up and down with a fixed time period set by a clock generator. During a subsequent conversion operation, the unknown voltage level analog signal is integrated over up and down ramps with the interval of integration being adjusted by the net offset error. Four integration intervals always are required for each measurement.
Substantially more circuitry is required for the dual- and quad-ramp A/D converter circuits than for the single-ramp circuit, and offset error correction in both the dual- and quad-ramp circuits is complicated.
The apparatus of the invention includes presenting transmittance digital data in the form of 16parallel binary bits. A transmittance value of 100% is represented by 2.sup.16 or 65,536 A to D units (ADU) while a transmittance value of 1% is presented as 655 ADU. A full scale analog transmittance signal is 10 volts with a least significant bit (LSB) weighing of 152 microvolts (10 volts.div.65,536 ADU).
Errors in excess of 10 millivolts may result from radiant energy detector dark current, preamplifier bias and offset voltages, plus digitizer offsets. Such an error can completely mask the low end of the transmittance data. The A/D converter circuit of the invention must substantially eliminate these errors.
Further, the apparatus of the invention must present the digitized transmittance signals to a central processor for conversion to absorbance data, processing, storage, etc. at a high rate. Lengthy periods between sample measurements are not available in which to perform error correction.
Errors related to gain are inconsequential in the A/D converter because they cancel one another during the conversion from digital transmittance to digital absorbance data. Thermal variations are negligible because the chemical reactions occuring in the cuvettes require a stable thermal environment which also is provided to the A/D converter circuitry.
The apparatus and method of the invention then must provide an A/D converter that is highly accurate, fast and that produces offset-error free transmittance data in digital format.